In recent years, from the standpoint of environmental issues and energy problems, fuel cells have been attracting attention. A fuel cell is a clean power generating device which generates electric power by a reverse electrolysis reaction of water using hydrogen and oxygen and discharges only water, which is classified into several species according to the kind of the electrolyte and among these, a solid polymer-type fuel cell operates at a low temperature and is most promising for automobile or electrical appliances. Such a fuel cell is fabricated, for example, by stacking single cells each comprising a polymer solid electrolyte, a gas diffusing electrode, a catalyst and a separator, whereby high-output power generation can be achieved.
The fuel cell having the above-described construction has a separator for dividing single cells. A flow path (groove) for supplying a fuel gas (e.g., hydrogen) and an oxidant gas (e.g., oxygen) and for discharging water (water vapor) generated is usually formed on the separator. Therefore, the separator is required to have high gas impermeability enabling complete separation of these gases and high electrical conductivity for decreasing internal resistance. Furthermore, the separator is required to be excellent in heat conductivity, durability, strength and the like.
For the purpose of satisfying these requirements, the separator for a fuel cell has been studied from both aspects of a metal material and a carbon material. The metal material has a large specific gravity, but is advantageous in that a thin separator can be fabricated because of its excellent mechanical properties, and moreover electrical conductivity is high. However, there is a problem in corrosion resistance. Studies are being made on the design of a surface treatment or composition that has excellent corrosion resistance.
On the other hand, many studies have also been made with regard to the carbon material, and examples of the material for the fuel cell separator include a molded article obtained by press-molding an expanded graphite sheet, a molded article obtained by impregnating a carbon sintered body with a resin and curing it, a glass-like carbon obtained by firing a thermosetting resin, and a molded article obtained by mixing a carbon powder and a resin and molding the mixture.
For example, Patent Document 1 discloses a complicated process of adding a binder material to a carbonaceous powder, mixing these under heating, subjecting the mixture to CIP molding (cold isostatic pressing), firing and graphitization, impregnating the obtained isotropic graphite material with a thermosetting resin and after curing, engraving a groove by cutting.
Also, Patent Document 2 discloses a technique of impregnating carbon powder- or carbon fiber-containing paper sheets with a thermosetting resin, stacking and press-bonding the paper sheets, and firing the stack, and Patent Document 3 discloses a technique of injection molding a phenol resin in a separator-shaped mold, and firing the molded article.
Such a material obtained through firing as in these examples exhibits high electrical conductivity and high heat resistance, but has problems that firing takes a long time to make the productivity low or that brittle destruction readily occurs. Furthermore, in the case where cutting is necessary, mass productivity is decreased and cost rises. For these reasons, the material will be difficult to spread in the future.
Meanwhile, reduction in contact resistance, which is a factor governing the electrical conductivity of the fuel cell separator is important. Some attempts have been made to reduce the contact resistance by devising a separator structure. For example, Patent Document 4 discloses a technique of coating a separator surface with a metal or carbon having high electrical conductivity, Patent Document 5 discloses a technique of applying an electrically conducting polymer to the surface of a molded article of an electrically conducting resin composition, and Patent Document 6 discloses a technique of applying an electrically conducting material to the surface or embedding it in the inside in the longitudinal direction.
Furthermore, gas shielding of the fuel cell is performed by using a gasket or a packing, and the outer circumferential part of the stack is treated for insulation so as to prevent a shortcircuit. For this purpose, a cell stack has been conventionally fabricated by individually prepared seal parts. Accordingly, the process of stacking several hundreds to several thousands of sheets takes a long time and is cumbersome. Considering these problems, for the purpose of reducing the number of parts as much as possible, there has been recently disclosed a technique, for example, in Patent Document 7, where a separator having a power generating part composed of a carbon-based material and an outer frame portion composed of an electrically non-conducting polymer material is processed by insert molding. In Patent Document 8, a technique where a separator and a gasket are integrated is disclosed.
In addition, Patent Document 9 discloses a technique of cutting the resin-rich layer (a layer rich in resin) on the separator surface to increase the area ratio of carbon powder on the surface, and Patent Document 10 discloses a technique of using rubber for the binder to enhance the adhesion on the contact surface and decrease the contact resistance.
[Patent Document 1] Kokai (Japanese Unexamined Patent Publication) No. 8-222241
[Patent Document 2] Kokai No. 60-161144
[Patent Document 3] Kokai No. 2001-68128
[Patent Document 4] Kokai No. 2001-196076
[Patent Document 5] Kokai No. 2002-8685
[Patent Document 6] Kokai No. 2001-52721
[Patent Document 7] WO 01/80339
[Patent Document 8] Kokai No. 2005-235631
[Patent Document 9] Kokai No. 2003-282084
[Patent Document 10] Kokai No. 2001-216977
As described above, the separator for a fuel cell is conventionally required to satisfy particularly high electrical conductivity, gas impermeability, strong corrosion resistance and low cost. Furthermore, a lightweight and compact separator capable of exerting its high performance in a limited space is demanded. In addition, a member which can dispense with a gas seal member and enables the fabrication of a fuel cell in a short time is demanded.
Regarding the separator, a resin mold-type carbon-based material not requiring a cutting step has been recently paid attention to, and is also being developed. However, although the amount of the electrical conductivity-imparting material packed needs to be greatly increased to express high electrical conductivity, reduction in the resin content is limited for maintaining moldability and sufficiently high electrical conductivity cannot be obtained. Also, the carbon-based separator has a low specific gravity as compared with metal and can advantageously contribute to lightweighting, but when the thickness is reduced, cracking readily occurs and the reliability of gas shielding decreases. In this way, a separator as thin as a metal separator is difficult to produce.
Furthermore, in the case of a conventional separator, as shown in FIG. 28, the flow path has a shape of symmetry from front to back and has a largely uneven thickness and when the material has a high viscosity, the processability is bad and a difference in density is readily produced between the thick part and thin part. Therefore, high flowability is required for molding a separator with excellent surface precision.
In the case of fabricating a fuel cell stack, a gasket or a packing is mainly used to prevent the escape of gas, but since the number of fabrication steps increases and the process becomes cumbersome, a structure not using such a gasket or packing is needed. Furthermore, the fuel cell stack is fabricated by applying an insulating treatment to the outer circumference so as to prevent shortcircuit, but elimination of this step is needed.
An object of the present invention is to overcome these drawbacks of conventional techniques and provide a lightweight, compact and high-performance fuel cell separator, a cell unit (single cell unit or short stack unit) for a fuel cell, and production methods thereof.